Phase modulation system



R. ADLER I 2,460,965

PHASE MODULATION SYS IEM Feb. 8, 1949.

. Filed Sept. 10, 1945 (I r 48 'Muliiplier MI Osc. aPower 'f'fi Amplifier Fig.!

Frequency Deviation Normal With Frequency Sharply Deviation Tuned Circuit Signal Frequency Signal Froquency (with constant signal intensity) coflsfaflf slqfldllnfeflslfy) Fig. 2 F /g. 4

' Modified Correction Network Norm I Transmission wim Sharply Correcfion Tuned Nefwo k Circuit Transmission Signal Freq 5W I I F Fig. 5

Slgna reouency INVENTOR. 3 ROBERT, A0 ER y M W6 mrronmsvs Patented Feb. 8, 1949 71: GTE-ICE:

2,160,965... RHAS'EjMODULATION-SYSTEM? Robert 1 Adler; Chicago, 111., assigmlrz topZenith Radio. Corporation; a corporation of-Jllinois Application.Septembet-m, 1945,Seria,l No. 615539? This inventionrelates to phase modulation carrier wave communication systems 0f1thetype in which, the phase of a carrier wave is modu-e latedv in accordance with intelligence to Joe trans mitted'in .such' a way as ;to produce frequency" modulation of the carrier wave. Normallygthe carrier wave amplitude in such a system is made constant, but it is within thescope of this inventionto produce. s c freq y. mo ul on of t ecarr eryvay ae omhani dby,amplitudem de lation ca sed.;by..thev.semesorY different intelliie gence, As the term is used herelne phaaemodi ulation, arrier. w v ommnn cat on;..systemff is. intendedto mean.-such,.a system...-

Phase modulation. arrien..wave sy .stems. ha e--. ena r ngedin themes; iwithmeans .f umodirj fyinga, sigi.1al.,or intelligence lto b'eetransmittedg insuch A away that, .when .th'e ..carrier.. wave is... modulated in phase in accordancewith the modi,-.- fied signal, the@-resu1ti-ng,. carrier wave is substantially .con'stantin amplitude. and is modulatedin frequency in accordance. with the unmodified; signals Therequiredmodification oiw tlie. signal is..a reduction in v.signalintensity .as. signal ill-'81. quency increases, so.that, foranunmodified sine; wave signal of ,increasingrfrequencyandof constant intensity, the: modified signal; has --a. =.con star-it product .ofrsignal frequency andr signal --intensity. In consequence; at high signal, free. quenciesthe signakintensityis .extremelysmall, andisin fact so small that. extraneoussnoise; voltages are comparable ;in magnitude and cause. phase modulation of the carrier. wave .-inamounts comparable to the carrier waves modulationi causedby the-signal itself- It has thereforebeen. extremely difficult toanakew the quality of: such: systemssufficiently; high; that is, to decrease gthe noise -.-inherent in :1 such systems. to a =1evel suffer, ciently low :that $116 gsignal is notgadyersely afi ieeted by-.-'su-ch noises.

u o se. is not yr n roduced;b 'r hermal; aeii' ionein; he ircuitsand:: VaQ1mm ubesmt t eas s eme utr lsaby electrostatic. andrelectroe ma e c. o p inezw' th external .sseunces ofeexe r ne u ..-zy l ases It m sibeenextreme y: ffi t to -shield.. sucl i-:sy;stems ;-.to 3 such; an -.ex;te.nt: esthat. s ch:ext aneonsiminduced'voltaeesibeeome neelie lb 'r: a i m m e i onsmth hee x remely smallssignal voltages-Turmoil have heretofore had to loe utilized.

It is accordingly an object of this ,inyentionto provide anmimproyed phase modulation" carrier wav .s stem. in whichethe.difiiaultiesrh retoiore xp rienced. withgundesiredi .n ise .;v.o1tag s1l. are eat y; m nimizednor.substantially, .ieliminated.

4 Claims. (01. 179-.-171.=5

It is a corollary object of the invention: to;

make possiblethezpractical usecctphase-modu-lae tion systems; in- -iwhich :the'; carrier wave a center frequency v may be: directly controlled 1 (as by a... piezoelectric oscillator): in applications. where. extremelyllownoise ievelis; essential and in some 2 cases --is required by-wgovernmental;:regulation;

tsisxanotl'i'er objecteofithis:invention to :pro: videssuch: animprovedsssystempinewhich:;the ele:-. merits aaresfevm-ands simple and: are readilyad.:v

justable for whig-h iqualitywperformance.

Theeieaturesaof: the. present scinven-tionare believed to be novelaaaitezsetzforth'iwithpareticularity- -in:-tied-appendediclaimssg Theapresent inventiomitselfgboth as-to its. organization-. ands manner .01:Eoperatiom together-withziurtherwoba jects :andzadyantages thereof may'ab est'ibesun-dere stood by referenceto the. following. description-- takemin connectionwith accompanying drawings I iITVVhiChI V Figure l illustrates .aradio.:transmitter:-com structed in: accordancewith-the invention; and T Figures :2 through 5 illustrate certain cha-rac zteri-stics: ofz phase modu1ation:- carrierwave: systems.

For the purpose of explaining with suflicient ciarity-th'e principles of-th'e invention; it -is-neces-- sa-ryfirst to describe in somedetail aeparticular phase -mcdulation-system" of' which' many parts fareknownbut cooperate with novel-elements and organizations thereof -'to utilize -=the present in vent-ion.-

In Figure -1 a" phase -modulation-radio carrier' wave transmitter --includes a highly' stableessillatonl'; a carrierwave phase"modulator 2 energized by the 0SCi11atOl""|, frequencyiimultipliers represented -by-the-rectangle 3 for .multip1yingthe-frequency ofFthe-phaSe modulated carrier.

waveifrom the.;,modu1ator' 2;? an additional .fre

groundythrou h;.abymassingscondenser llLlfThe quency-multiplier andaifinalkpower amplifier 7 highly stable oscillator l is connected between ground and one terminal of the tuned circuit 8, 9, which is resonant to the frequency of the output of the highly stable oscillator I. The phase modulator 2 also includes three electron discharge amplifier devices II, I2 and I3 which respectively amplify the carrier wave in phase quadrature with each other. These carrier waves are derived from the tuned circuit 8, 9 by connection of the first control electrode l4 of the device II through coupling condenser I5 to one terminal of the tuned circuit 8, 9, by connection of the first control electrode I5 of the device I2 through coupling condenser I! to the other terminal of tuned circuit 8, 9, and-by connection of the first control electrode I8 of device f3 to a point between a resistance I9 and a condenser 29, the resistance I9 and condenser being connected serially between the terminals of the tuned circuit 8, 9, the impedances of the resistance I9 and condenser 23 being equal at the frequency of the carrier wave from oscillator I. By such connections, the carrier wave voltage is impressed on control electrode I4 in phase with the voltage from oscillator I. A carrier wave is impressed on control electrode IS in opposite phase to that from oscillator I, and a carrier Wave is impressed on control electrode I8 which lags the carrier wave voltage from oscillator I by 90, all of the carrier wave voltages on the three control electrodes I4, I6 and I8 being substantially equal in intensity.

The discharge devices [I l2 and I 3 are suitably connected to act as amplifiers for the carrier waves impressed on their control electrodes, the respective anodes 2 I, 22 and 23 being all connected together and through a parallel resonant circuit including an inductance 24, a, resistance 25 and a condenser 26 to the positive terminal of a suitable source 27 of operating current, the negative terminal of the source 21 being grounded. Cathode'28' of the discharge device II is connected through a by-passing condenser 29 to ground, and is connected through biasing resistances 30 and 3| connected in series to ground. Cathode 32 of the device I2 is connected for high. frequency current through by-passing condenser 33 to ground and is connected through a, biasing resistor 34 and through biasing resistor 3| to ground. The control electrodes I4 and I6 are connected to the respective terminals of the secondary 35 of a transformer, the center tap of the secondary 35 being grounded so the discharge currents flowing through the resistances 39, 3| and 34 produce bias voltages between the cathode 28 and control electrode I4, and between the cathode 32 and control electrode I6. These bias voltages may be individually adjusted by suitable adjustment of the resistances 30 and 64. The cathode 36 of discharge device I3 is connected to ground through a biasing resistance 31 by-Dassed with a condenser 38, and control electrode I8 of discharge device I3 is connected to ground through a large resistance 39 so the discharge current flowing through the resistance 31 producesa biasing potential between the control electrode 18 and cathode 36. This biasing potential may be adjusted by suitable adjustment of resistance 31. Resistance 39 is large with respect to resistance I9 sothat it does not produce an appreciable alteration of the phase angleof carrier wave voltage between control electrode I8 and ground. The respective screen electrodes 40, 4Iand 42 of the discharge devices II, I2 and I3 are all connected together and through a, resistance 43 to the positive terminal of source 21. The screen electrodes 40, 4| and 42 are also connected to ground through a by-p-assing condenser 44 which has low reactance at the lowest frequency of signals from signal source 6.

The respective gains of the discharge devices II, I2 and I3 are adjusted by suitable adjustment of resistances 39, 34 and 31 so that they are substantially equal, in order that the amplified carrier wave potentials through each of those discharge devices shall appear across the tuned circuit 24, 26 in substantially equal intensities, although in phase angle differing respectively by substantially The reactance of the secondary 35 connected between control electrodes I4 and I6 is high at the carrier wave frequency. Since the carrier wave potentials amplified by devices II and I2 are substantially opposite in phase, the connection between the anodes 2i and- 22 causes the substantially complete cancellation of carrier wave potentials amplified through devices II and I2 so long as no potential appears in the secondary 35. Therefore, under such conditions, carrier wave potentials appearing across the tuned circuit 24, 26 are substantially only those amplified through device I3.

The carrier wave potential appearing across the tuned circuit24, 26 is transferred to the frequency multipliers 3 by reason of magnetic coupling-between the inductance 24 and an inductance 45 connected across the input terminals of the frequency multipliers 3. A condenser 46 is connected in shunt with the inductance 45 and is adjusted to resonate withthe inductance 45 at the carrier wave frequency from the stable oscillator I, which is also the resonant frequency of the tuned circuit 24, 26. A resistance 41 is also connected in shunt with the inductance 4 5, and the resistances 25 and 41 respectively damp the tuned circuits 24, 26 and 45, 46 so that those tuned circuits are rather broadly resonant to the carrier wave frequency.

'An inductance 48 is connected between the output terminals of the frequency multipliers 3 so that a carrier wave potential of suitably multiplied frequency appears across it. This inductance 48 is untuned and is magnetically coupled with an inductance 49 with which a condenser 5B- is connected in shunt, and with which a resistance 5| is also connected in shunt. The parallel combination of inductance 49 and condenser 50 is resonant'at the multiplied carrier wave frequency from the frequency multipliers 3, and the resistance 5| produces a critical amount of damping in the resonant circuit 49, 59 for a purpose to be described later.

The carrier wave of 'multiplied frequency induced across the tuned circuit 49, 59 is impressed between the input terminals of the frequency multiplier and power amplifier 4, in which the frequency of such carrier wave is further multiplied by the factor N to reach the desired final carrier wave frequency at which'the desired frequency deviation DA is achieved and in which the power amplifier raises the power level of that carrier wave of multiplied frequency to a power suitable for radiation. The output terminals of the frequency multiplier'and power amplifier 4 are connected to the antenna 5 so that the high power frequency multiplied carrier wave is radiated into space.

V Signals fromthe signal source 6 are caused to producephaseshiftofthe carrier wave in the phase modulatorv 2 by transmission through the following circuit: The terminals 52 of source 6 are connected respectively :tQ the rterminals mf a resistance53 and a resistanc'e 54; connected serially, :acondenser 55 "beingzeonnected in with the resistance 53. lI hat outputterminail fl of the signal source 6 which is connected to resistance 54 is grounded, and the control elec: trode 55 of the electron discharge amplifier device I is connected to a point between resistances 53 and *54. The cathode'5l of the device I is connected through a biasing resistance 51! to round, and a large by-passine ndenser 58 is connectedi'in-shunt-with thejresistancefia; The anode 60 of the device :1 is;conne-cteditl1rough the primary 6| of a transformer to the ipositiue terminal of the source 21, The transformerprimary 6-1 has high reactance at allsignal frequencies irorn the source 6 :and the discharge device 1 so connected amplifies signals fromii' he source 6 and impresses them across the transformer primary -61. The network including resistances 53 and 54 and condense-1"55 is efie'ctive to produce an initial modification of signals from-the source 6, commonly termed preemphasis? To produce such an 'effect, 'thi s network is so adjusted as to produce for asirie wave signal from the source 6 of increasing frequency from a, certain upward-minimum rrequency'approximately a 6 decibel "increase in intensity for each octave --incr,ease in frequency of :the signal.

Such preemphasis" is commonly utilizedfin phase or frequency modulation-transmission systems for the purpose of minimizing inoisej'uolts ap e rin the da rier'qwai'e fm fl ata and in subsequent-portions of thegsystem,

network is included in thissystem orflyfor 'the purpose of producing a standandtype of menulation-with preemphasisz f- The transformerprimary 'fi f'is suitablymagnetically coupled for signal frequencies with 'a; secondary 62 of which the center tap is grounded? One terminal of the secondary 62"is connected through the resistance S3 -toone terminal of the primary winding -64 ofthe transformer whose sec-' ondary is'the winding: 35. The other termignalpf .the transformer secondary r52 is connected through a resistance 65 .tovthe other terminal f the transformer primary 54. A condenserjfi and a resistance 61 are connectedyserially between the terminals of thetransiormer primary 564 and th network includingresistances; 63, and ,8! and'condenser 66, together-with thelcriv maladjustment of damping resistance-5 the tuned circuit 39, 50 produces thelg-reatidegree of noise suppression achieyed by this iiwention.

Let j represent. the lirequency or ari'unmedulated' wave. 7 $11,011 a wavemay he modulated in ha e bye si a e of lowe ir encv etit a m mum has excu sion K u' dif adians). Such a phase modulatedfjjaye may also be re arded .as a wa or va ing iredu nc Let the maximum deviation of ireduencyof such modulated wave from fo be called 11);

Then: i f.

et: a a D p e en correspcndinecharacteristicscfsthe wave at the input :to the he quency multipliers 4. p Then similarly: V

fm a which is a statement, illustrated line 68 in Fig. 12, that withlcjonstant-phase shift chin-other Words. constant signal intensity-"at the input-terrninals of the phase modulator 2,: there is a direct proporti nality betweenDu-and E. f e r (2):.

certain technical and legall reasOnsJit isius'uaI- tO design ;:such :a system f or ca, maximum-deviation Fis :the same Ifor 'all signal frequen cies'em To satisfy that requirement it. is-nec- 7. sum! finr-ithetphasessexcursion 41, zcorrespondingrtoimaxtimum-sig-mthintensity. I

.L'Ihis means rthatzattenuation, :shouId lbe 1131-0;

:duced :in ";the signal (before it ..is impressed won fiihasafmodulator 2"), so that theainterisityzof isi'g- ,nal-limpressedqon :modulator 2,1 with constant-;-intensity signal at source'fi, is inversely proportional, to signal irequency i This s relationltis i-illustratediby .thyperbola 69 and its dotted end '10 in Fig. 3. A true hyperbolic relation of that.na-' 1 time isf-notlreadilyj attained and the correction Y network-designed to :produce.:such attenuation, is :in' practice arranged .to deviate frem tthebtrue .hyperhcl-a (at extreme low signal; frequencies according totheendiportion H of curve .69..

Let new re'presentomaximum:deyiation'zat antenna 5 corresponding to Dmx :at thepinput to emultip ier 4,, and N; the factorvby which rre is multiplied in multiplier 4.

' A(sm-m- A proper choice ofZN is possible so that $1 is small compared to .'(1 -1) at least at higher signal frequencies. a

New" assuming that for a particular signal ire-i quencyfm this condition that 1 l is fulfilled,

the grave of frequency In, phase modulated by the signal or frequency, fm may be closely apprcxig :mated by the vcombination of an unmodulated or center j'rgequency wave, of irequencyafsand two side hands, each of constant frequency,- 5 phase andampiitude, of respective frequencies f o-Tim I and; 15-1, and of amplitudes. /2 withthe amplitude-of the unmodulated waye being regarded asunity.

When such through, aresonant system, the amplitudes of the center -.frequency -wave I f0 and the sidebands joqt-fmfilld'fbrfm are changedinrelation to each othergasdictated by: the frequency character -istic or the resonant system. It has been com- Q-monpractice tol make the resonant systems following" the phase modulator 2 of sufficient-band width that. substantially no attenuation is pro duced in either of the'sidebands fo+fm' and fofm at any time. a

According to the V sharply;- resonant system is introduced between irequency multiplier 3. and frequency multiplier d transmits uniformly a frequency band I of widtlrZ-E centered: around 10, but symmetri- .ically attenuates frequencies outside "this *band.

:11 corresponds toa signal-"frequency-fm which" is I :substantiallylcwer than the upper limit 01 signal frequencies robe-transmitted 'throughthesystem, but enough that'tpr is still much less thanunityi Let .afsignal freduencyfm' higher than he att-enuated'by a factor a.v After passing through era-attenuating system the sideband ampli- -tudes-arereducedifrom- L .1

modulated wave is transmittedpresent invention; a more frequencies the deviation phase modulator 2 7 Theresult isthat the sidebands after such attenuation are as though they had been caused by a. signal of correspondingly smaller intensity. At the output of multiplier 4 the deviation is reduced by the factor. a. This. eifect takes place only when signal frequency fm exceeds the limit corresponding to F, so that the'sidebands-fall within the regions of attenuation. For lower signal is not affected.

The previous discussion is actually a simplifilcation of fact in assuming thatamplitude' is-no less at foi'F that at is. In practice- Where a single turned circuit is used to produce the desired attenuation as the circuit 49, 50, Si in Fig. 1, the damping resistor 5i is adjusted so that-the :amplitude at fail is 70% of the amplitude at fo- PFor frequencies diifering from f by amounts greater than F, expressed as foiKF (where IK 1) the. attenuation a .atoKJ is substantially equal With such adjustment of the circuit 49, 50,' the :curve 12 (Figure 4) illustrates the relation between signal frequency fm (with constant phase excursion or signal intensity atthe; input to modulatorl) plotted as abscissa-and the resulting frequency deviation D, measured at the input to multipliers 4, plotted as ordinate. This relation maybe expressed in-te'rms of Equation 2 when it is understood that for a signal q n y m=KF (where' K l) side band attenuation causes approximatelya reduction of the devi- :atio'n by factor K.

meals: 7 v V q 1 K -I I V v p (5) For the signal frequencies fm less thanF'Equation 2'remains approximately'truer v According to the invention, the correcting network 53; 65, "86is so modified byaddition' of re :sistaneelil that the voltage"introduced into the is inversely proportional to jm :so long acne is smaller than F but remainssubstantially "constant when fin is larger than F. In the practical network '53, 65, 65,197 in Fig. 1, the transmission at jm=F is not actually equal to the constant amount of transmission at much'higher frequencies, but is 141% as much, so as'to compensate for the described shape of attenuation characteristic of the turned circuit; 49', 5 fl,- 5|. This relation is illustrated by curve 'l3'inFig. 5, in which'signal frequency finis plotted as abscissa against network transmission plotted as ordinate. 1 In such a system, the resulting phase excursion for full deviation frequency'below DA (max) Expressed in another way, the effect ofextram. eous voltages and noise on the system is reduced by the factor K, which in a practical case may be made quite large with acorresponding. large reduction in the eflect, of high frequency noise. 1 In fact, such a system is readily constructed, Wit out any special critical adjustments'jusing .conventional phase modulating meansand frequency multipliers to transmits, high fidelity signa1 of excellent broadcast quality where such'a system without the present invention is incapable of such performance. 1 Choice of the frequency limit F is based on. -the fact that the representation of a.-'phase modulated wave by acarrier wave and only two side never is reduced at any signal;

frequency other than and modifications may be" bands is valid only where phase excursion is much;

lessthanone radian 1). Sidebands of sub- T0 make this phase excursion on small enough, F must be chosen several times larger than Di.

v D itself may be made relatively small by making N large, which means carrying out a, large fraction of the total frequency multiplication in multiplier; 4 following the resonant system 49, 50, 5!. However, if N- be made too large (by having too'little or no frequency multiplication in multiplier- 3 ahead of the resonant system 49, 50, 5|) noticeablenoise effects appear in the first stages of frequency multiplication following the resonant system. A thus defined for ratio. 7

Then for a low frequency signal fm,

practical upper limit for N is any particular final deviation where 41 is large; many sidebands appear, covering substantially the entire band from fo-D1 to f+ r and this band of frequencies then is much wider than that between the first two sidebands fo-fm andjft-l fni, but since F is chosen several times larger-than Dl, even that wide band from foDi to @{Di is transmitted without substantial attenuation; so that the transmission of low frc quenoysignals remains substantially unafiected. Purely, by wayv of example, let it be assumed that a transmitter of this typebe desired for operation at 96 inegacycles with a maximum deviation DA (max) of '75 kilocycles. The various portions of such a system may be adjusted as follows for satisfactory operation.

Then: I

i 250 kiiocycles D1==. cycles F=l500 cycles and it follows that the timed circuit 49, so, 5: is

resonant at 250"kc. and is adjustedto have a fig- .ure of merit of 83, and that the reactance of condenser B 6 is'equal to resistancetl at 1500 cycles. For noise at 300;} cycles,an improvement in signal to noise ratio of about: six decibels is attained,

' plicated networks to replace tuned circuit 49,. 50, 5 l and correspondingly more complex compensating circuitsto replace circuit.63, 65, B6, 61. For example, itlmay be desired to provide a relation between side band attenuation and signal that described with a corresponding compensating network to obtain any desired transmission of the signal. Further, the network 53, 54, 55, may be omitted and the network 63, 65, 86, 67 modified to carry out, in addition to its described function, the function of providing preemphasis.

While particular embodiments of the present invention have been shown'and described, it will be obvious to those skilled in'the art that changes 7 made without departing from this invention in its broader aspects, and,

therefore, the aim in the appended claims is to cover all such changes and modifications as fall within the true spirit and scope of this invention.

I claim:

1. In combination, a carrier wave source, a frequency multiplier for multiplying the frequency of the carrier wave from said course, a modulating signal source, the signals from said signal source lying in a band of frequencies including a first and a second range contiguous with each other, said second range including higher frequencies than said first range, a modulator connected between said carrier wave and signal sources and constructed to vary the phase of the carrier wave from said carrier wave source as a function of the instantaneous amplitude of the signal from said signal source, said modulator including a filter circuit constructed to cause the maximum phase excursion of said carrier wave to be substantially inversely proportional to the frequency of modulating signals lying within said first range of frequencies and to be less than one radian for modulating signals lying in said second range, said carrier wave having an unmodulated frequency, and a tuned circuit connected between said modulator and said frequency multiplier and constructed to attenuate side band components produced by signals lying in said second range directly as a function of the magnitude of such signal frequency, and to pass without substantial modification carrier waves modulated by signals in said first range.

2. In combination, a carrier wave source, a frequency multiplier for multiplying the frequency of the carrier wave from said source, a modulating signal source, the signals from said signal source lying in a band of frequencies including a first and second range contiguous with each other, said second range including higher frequencies than said first range, a modulator connected between said carrier wave and signal sources and constructed to vary the phase of the carrier wave from said carrier Wave source as a function of the instantaneous amplitude of a signal from said signal source, said modulator including a filter circuit constructed to cause the maximum phase excursion ofsaid carrier wave to be substantially inversely proportional to the frequency of modulating signals lying within said first range of frequencies and to be less than one radian for modulating signals lying in said second range, said carrier wave having an unmodulated frequency equal to its center modulated frequency, and a tuned circuit connected between said modulator and said frequency multiplier, resonant at said center frequency and constructed to attenuate side band components produced by signals lying in said second range directly as a function of the magnitude of such signal frequency, and to pass without substantial modification carrier waves modulated by signals in said first range.

3. In combination, a carrier wave source, a frequency-multiplier for multiplying the frequency of the carrier wave from said source, a modulat- 6 ing signal source, the signals from said signal source lying in a band of frequencies including a first and a second range contiguous with each other, said second range including higher frequencies than said first range, a modulator connected between said carrier wave and signal 7 sources and constructed to vary the phase of the carrier wave from said carrier wave source as a function of the instantaneous amplitude of a signal from said signal source, said modulator including a filter circuit constructed to cause the maximum phase excursion of said carrier wave to be substantially inversely proportional to the frequency of modulating signals lying within said first range of frequencies and to be less than one radian for modulating signals lying in said second range, said carrier wave having an unmodulated frequency equal to its center modulated frequency, and a single tuned circuit connected between said modulator and said frequency multiplier and clamped to attenuate side band components produced by signals lying in said second range directly as a function of the magnitude of such signal frequency, and to pass without substantial modification carrier waves modulated by signals in said first range.

4. In combination, a carrier wave source, a frequency multiplier for multiplying the frequency of the carrier wave from said source, a modulating signal source, the signals from said signal source lying in a band of frequencies including a first and a second range contiguous with each other, said second range including higher frequencies than said first range, a modulator connect-ed between said carrier wave and signal sources and constructed to vary the phase of the carrier wave from said carrier wave source as a function of the instantaneous amplitude of a signal from said signal source, said modulator including a filter circuit constructed to cause the maximum phase excursion of said carrier wave to be substantially inversely proportional to the frequency of modulating signals lying within said first range of frequencies and to be less than one radian for moduatingsignals lying in said second range, said carrier wave having an unmodulated frequency equal to its center modulated frequency, and a single tuned circuit resonant at said center frequency connected between said modulator and said frequency multiplier and damped to attenuate side band components produced by signals lying in said second range directly as a function of the magnitude of such signal frequency, and to pass without substantial modification carrier waves modulated by signals in said first range.

ROBERT ADLER.

REFERENCES CITED The following references are of record in the file of this patent UNITED STATES PATENTS Number Name Date 2,347,458 Brown Apr. 25, 1944 2,347,459 Goetter Apr. 25, 1944 2,396,688 Crosby Mar. 19, 1946 Assistant Commissioner 0 Patents.

Certificate of Correction Patent N 0. 2,460,965. February 8, 1949.

ROBERT ADLER It is hereby certified that errors appear in the printed specification of the above numbered patent requiring correction as follows:

Column 3, line 59, for the numeral 64 read 34; column 6, line 44, for f f it read f +f,,,; line 57, same column, for the Word and read or; column 10, line 43, claim 4, for moduating read modulating;

and that the said Letters Patent should be read with these corrections therein that the same may conform to the record of the case in the Patent Oflice.

Signed and sealed this 28th day of June, A. D. 1949.

THOMAS F. MURPHY, 

